<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .container {
      display: flex;
      flex-direction: row;
      width: 600px;
      height: 100vh;
    }
    .bg-red {
      background: red;
    }
    .bg-green {
      background: green;
    }
    .flex1 {
      flex: 1;
    }
    .height100 {
      width: 100%;
      height: 100px;
    }
  </style>
</head>
<body>
   <div class="container">
    <div class="flex1 bg-red">flex1</div>
    <div class="height100 bg-green">100px</div>
   </div>
</body>
</html>